What if the sibling only allows supervised phone calls and visits and will not allow her to visit any of the other children. When visits are made mom always asked for food, tells stories of neglect, never has any clothes that fit or toiletries. When questioned you are banned from contact. He just took her to have a new will made and have banned the five of us from family home. Hospital filed case for neglect but sent her back to home.
He is clearly holding her hostage and, if the hospital has filed neglect charges, and she has complained about being hungry, you really have a problem. (I'd have beat his door down, but the that's just me.)
Get the law involved immediately. All 5 of you take the sheriff or police and go to the home of your brother. One of you need to be able to move mom in with you when she is removed from his home.
Contact an elder lawyer also. What your brother is doing is against the law. He could be charged with kidnapping.
I just don't get WHY you've let this go on so long.